Summary of work: Sonodynamic therapy is a promising new modality for cancer treatment based on the synergistic effects of cell killing by a combination of sonosensitzer and ultrasound. Ultrasound can penetrate deeply into tissue and can be focused in a small region of tumor to activate non-toxic molecules (e.g. porphyrins ) thus minimizing undesirable side effects. The experimental evidence suggests that sonosensitization is due to the chemical activation of sonosensitizers inside or in close vicinity of hot collapsing cavitation bubbles to form sensitizer-derived radicals either by direct pyrolysis of the sensitizer at the water-gas interface or due to the reactions of hydrogen atoms and hydroxyl radicals formed by the pyrolysis of water. The free radicals derived from the sonosensitizer (mostly carbon-centered) react with oxygen to form peroxyl and alkoxyl radicals. Unlike OH radicals and H atoms which are formed by pyrolysis inside cavitation bubbles, the reactivity of alkoxyl and peroxyl radicals with organic compounds in biological media is much lower and hence they have a higher probability of reaching critical cellular sites. Recently we have succeeded in spin-trapping the carbon radicals formed during the sonolysis of aqueous solutions of various porphyrins . The surfactant properties of solutes play an important role in the sonochemistry of aqueous solutions. Recently, it has been shown, for relatively low molecular weight surfactants, that these effects can be correlated with the Gibbs surface excess of the solute. We investigated whether this correlation is valid for relatively high molecular weight surfactants, such as sonosensitzers, by considering the Gibbs surface excess and the possible role of the dynamic surface tension using model surfactant molecules, with well characterized surfactant properties. The results show that the Gibbs surface excess of high molecular weight surfactants does not correlate with the extent of decomposition following sonolysis in aqueous solutions. Instead, the decomposition of surfactants depends on their chemical structure and their ability to equilibrate between the bulk solution and the gas/solution interface of cavitation bubbles.